1. Field of the Invention
The present invention relates to a Group III nitride semiconductor light-emitting device whose light extraction performance is improved by forming irregularities on a sapphire substrate included in the device.
2. Background Art
In recent years, Group III nitride semiconductor light-emitting devices have begun to be used in general illumination applications, and have been strongly required to exhibit improved light extraction performance. Japanese Patent Application Laid-Open (kokai) No. 2003-318441 discloses a method for improving the light extraction performance of a semiconductor light-emitting device, in which irregularities are formed on a sapphire substrate. In the case of a semiconductor light-emitting device including a flat sapphire substrate having no irregularities, light propagated in the device in a direction horizontal to the substrate is confined in semiconductor layers and is attenuated through, for example, repeated multiple reflection. In contrast, in the case of a semiconductor light-emitting device including a sapphire substrate having irregularities, light propagated in the device in a direction horizontal to the substrate can be reflected or scattered in a direction perpendicular to the substrate and can be extracted to the outside, whereby light extraction performance can be improved. Such irregularities may have, for example, a stripe pattern or a dot pattern as viewed from above.
Meanwhile, Japanese Patent Application Laid-Open (kokai) No. 2010-171382 discloses that irregularities are formed on a transparent substrate to output a light from the side surface of a substrate as well as to suppress the total reflection of light in order to improve poor light extraction performance due to the total reflection of a light emitted from a light-emitting layer at an interface between a sapphire substrate and a nitride semiconductor.
Further, Japanese Patent Application Laid-Open (kokai) No. 2010-225787 discloses that mesas are formed at a boundary between a sapphire substrate and a nitride semiconductor in order to reduce difficulty in extracting the light to the outside due to the total reflection at the interface, as in the Japanese Patent Application Laid-Open (kokai) No. 2010-171382.
However, there is no suggestion in any of the above references that mesas or dents formed on the sapphire substrate generate a light interference pattern. Moreover, there is no suggestion that light extraction performance is improved using the light interference pattern.